The study of cancer risk is a major component of environmental epidemiology. The Epidemiology Branch emphasizes the development of methods for assessing environmental exposures and, where feasible, uses biochemical measures of exposure or disease markers to evaluate cancer risk. Current studies focus on cancer risk from passive exposure to cigarette smoke and risk factors for leukemia. Studies in the Branch have identified associations between passive exposure to cigarette smoke during childhood and adulthood and risk for cancer at several sites in adulthood. Current studies are aimed at confirming some of these associations, documenting passive exposure to cigarette smoke, and understanding mechanisms of cancer risk. A prospective study of mortality and cancer incidence among persons who lived with smokers in 1963 has been conducted. In another study, indicators of mutagenesis and genotoxicity were measured in nonsmokers, passive smokers and active smokers. In addition, data from the Branch's study of cancer risk from passive and transplacental exposure to cigarette smoke were used to evaluate the quality of retrospective questionnaire data on passive smoke exposure and to explore a possible association between passive cigarette smoke exposure and early age at menopause. Risk factors for acute leukemias in adults are being evaluated in a cooperative case-control study. The study aims to identify risk factors for cytogenetically defined subgroups of acute leukemia. Some risk factors, such as exposure to solvents, may be more strongly associated with cytogenetically defined leukemia subgroups. Therefore, the results of bone marrow cytogenetics being done as part of other cancer treatment studies are being used to classify patients into potentially etiologically distinct subgroups. In a related project, the relationship between low-level background irradiation and leukemia risk is being explored in an ecologic study of radon in ground water and leukemia mortality.